This invention relates to a process for the low-temperature electrolysis in manufacturing aluminum, which produces aluminum by electrolysis of aluminum chloride.
From an industrial point of view, aluminum has been produced to date by the Hall-Heroult Process invented in 1885 which comprises melting alumina in cryolite and electrolyzing the molten alumina.
With the latest developments in electrolytic techniques, the electric power for electrolysis according to the above-mentioned process has been reduced to 14,000 KWH per ton of aluminum. Nevertheless, the efficiency of the process in terms of energy consumption for the reduction of aluminum remains at a mere 24% as 76% of energy is consumed in keeping the electrolytic bath at about 970.degree. C.
In view of the fact that lowering the temperature of the electrolytic bath reduces the thermal energy required for keeping the bath temperature stable, Aluminum Company of America developed a process for electrolyzing an electrolytic bath composed of 50% of NaCl, 45% of LiCl and 5% of AlCl.sub.3 at a temperature of 700.degree. C. ("A Revolutionary Alcoa Process," Business Week, Jan. 20, 1973, J. D. Harper: Eng. Min. J., 174, No. 2(1973), 30). The process, however, required special and expensive materials for the electrolytic cell such as silicon nitride, etc. due to high corrosiveness of AlCl.sub.3.
Studies on the electrolysis of an electrolytic bath composed preponderantly of AlCl.sub.3 at low temperatures in the range of from 120.degree. C. to 250.degree. C. have been conducted (Plotnikow et al: Z. Electrochem., 37(1931), 83; Engelhardt: Handb. Techn. Electrochem., 3(1934), 384; Czochralski, Mikolajczyk: Wiadomosci, Inst. Metallurg., 2(1935), 31; and Midorikawa et al: Electrochem., 24(1956), 562). However, anodes of carbon or graphite, which are the materials generally used for anodes in this field, disintegrate when applied to this method of electrolysis and therefore cannot be used. Thus the result of these studies was simply an electrolytic process for aluminum refining wherein aluminum of high purity is educed on the cathode by using aluminum of ordinary grade as the anode. This process has so far failed to find utility in practical applications. Midorikawa et al developed a technique for improving the electric depositions of aluminum on the cathode plate by adding lead chloride to the electrolytic bath. However, even when their technique is carried out under the most favorable conditions, the current density used in the formation of the electrically deposited layer cannot be raised above 0.5 A/dm.sup.2 and the thickness of the resulting layer is about 0.1 mm at most. If the current density is raised above 0.5 A/dm.sup.2, the educed aluminum is not deposited on the cathode plate and therefore cannot be collected. Since the current density is too low and the electrically deposited layer is too thin, their technique has not been reduced to commercial application.
The object of the present invention is to provide an improvement in a process for educing aluminum on the cathode by the electrolysis of a bath composed preponderantly of aluminum chloride, which improvement is capable of industrially producing aluminum in an electrolytic bath of low temperature.